1. Field of the Invention
The present invention relates to a valve for regulating the flow of a liquid or gaseous fluid, the valve being of the type including a body having an inlet and an outlet for the fluid, a partition which separates the inlet from the outlet and which includes a cylindrical bore having a circular seat at one end thereof, a valve member that is movable in the axial direction of the circular seat between first and second positions in which the valve is respectively closed and open, and control means functionally connected to said valve member in order to bring it into any desired axial position between its first and second positions so as to regulate the flow of the fluid, said valve member including a first portion which serves as a closure element when the valve member is in its first position, and a second portion which is engaged in said bore and which is shaped to regulate the flow of the fluid at a desired rate depending on the axial position selected for the valve member between its first and second positions.
2. Description of the Prior Art
Known valves described in documents U.S. Pat. No. 3,108,777 and FR-A-2 650 362, for example. In those two documents, the first portion of the moving valve member is substantially in the form of a disk and is connected via a rod to the moving core of an electromagnet serving as control means. When the coil of the electromagnet is not excited, the valve member is held by a return spring in its first position or valve-closed position. In known electrically-controlled valves, the second portion of the valve member is constituted by a part which is a body of revolution about the axis of the rod, e.g. a truncated cone, and which is fixed securely to the first portion of the valve member. Thus, by giving a predefined shape to the generator lines of the surface of revolution which constitutes the outside surface of the second portion of the valve member, it is possible, in principle, to obtain any desired flow/displacement characteristic when the valve member is displaced axially. However, in practice, in order to obtain the desired flow/displacement characteristic in fact, a valve member of that structure requires the axis of the surface of revolution to be accurately centered or aligned with the axis of the circular seat of the valve. This requires high precision when manufacturing the seat and the valve member, when assembling the two elements together, and when guiding the control rod of the valve member, and as a result such a valve is relatively costly.
Furthermore, in certain fields of use, e.g. in the field of gas boilers where valves of the above-mentioned type are used as means for regulating the flow of fuel gas sent to an air/gas mixing chamber preceding the burner of the boiler, in order to modulate the flow as a function of the instantaneous heating power required by the boiler while simultaneously keeping the flow of gas and the flow of air in a predetermined ratio, it is often desirable for the flow of gas to be regulated in a manner that is very fine and smooth, especially in the low-flowrate range, i.e. when the first portion of the moving valve member is in an axial position that is very close to the circular seat of the valve. In other words, in the low-flowrate range, it is desirable that any given axial displacement of the valve member causes only a small variation in the flow of fluid passing through the valve, such that the flow/displacement characteristic of the valve has a shallow slope in the low-flowrate range. In practice, this is difficult to obtain with a known electrically-controlled valve of the above-mentioned type, in which the second portion of the moving valve member has an outside surface in the shape of a truncated cone.
Furthermore, in known electrically-controlled valves of the above-mentioned type, it is usual to excite the electromagnet by means of pure or approximate AC so as to eliminate, at least in part, the magnetic hysteresis that the electromagnet would exhibit if it were to be excited by DC. However, exciting the electromagnet with AC causes the valve member to oscillate, which imparts oscillations into the flow and the pressure of the gas downstream of the electrically-controlled valve. Such oscillations are harmful to the flame of the burner, especially in the low-gasflow range. To avoid such oscillations, it is thus necessary to associate a damper, e.g. a hydraulic damper, with the moving equipment constituted by the electrically-controlled valve member and by the moving core of its electromagnet, as described in the above-mentioned American and French patents. It should also be observed that in the low-flowrate range, when the moving valve member is very close to its circular seat, the oscillations of the valve member can also create noise as a result of the repeated impacts of the valve member on its seat.
That is why the hydraulic damper must be very effective, and as a result it is relatively complex to make. In particular, the radial clearance between the piston and the cylinder of the hydraulic damper must be very small, which requires high mechanical precision in the machining and assembly of the two elements. Furthermore, since the presence of the hydraulic damper greatly reduces the displacement speed of the moving valve member, the electrically-controlled valve has a response time that is long and a check-valve must be provided that is connected hydraulically in parallel with the damper to enable the electrically-controlled valve member to be closed rapidly whenever the electrically-controlled valve is also used as a safety valve. In that case, the safety valve member must be capable of being closed in less than one second.
For all of the reasons mentioned above, electrically-controlled valves of the same type as those described in patent U.S. Pat. No. 3,108,777 and FR-A-2 650 362 are described in patent U.S. Pat. No. 3,108,777 and FR-A-2 650 362 are relatively costly. Furthermore, although the use of pure or approximate AC for exciting the electromagnet enables the magnetic hysteresis of the electromagnet to be eliminated at least in part, it does not eliminate mechanical hysteresis resulting from friction. Since the addition of a hydraulic damper to the electrically-controlled valve has the effect of increasing friction, such a damper thus increases mechanical hysteresis, and the greater the effectiveness of the damper, the greater the mechanical hysteresis it has. Unfortunately, for the same reasons as those described in document FR-A-2 650 362, mechanical hysteresis is a problem when the electrically-controlled valve is used as a regulating element in a temperature regulation loop, since it deteriorates the accuracy of the regulation.
Valves comprising two portions are also known from patents CH 581 284 A (KLEIN SCHANZLIN & BECKER AG), FR 619 503 A (P. PASCALE), and U.S. Pat. No. 4,125,129 A (BAUMANN HANS D). However, the two portions are not detached, but are securely connected to each other.